A step height standard is useful for calibrating surface profiling instruments. One such standard is disclosed by Peter Z. Takacs and Eugene L. Church in an article entitled "A Step-Height Standard for Surface Profiler Calibration" in SPIE Vol. 1995 Optical Scattering (1993), pp. 235-244. That standard includes a flat region with a single central step anisotropically etched therein and also includes two laminar profile bar patterns of 50 .mu.m and 10 .mu.m pitch. The single step is useful for calibrating optical profilers, while the bar patterns are useful for calibrating stylus profilers. The bar pattern ensures that the stylus probe will find an edge on the first try regardless of its lateral position. The peak-to-valley height of steps are approximately 150 nm.
Recently, techniques have been developed to make smaller scale vertical features. One such method for use in making calibration standards is described in U.S. Pat. No. 5,599,464 to Laird et al., assigned to the assignee of the present invention. With that method, features having vertical heights on the order of 10 .ANG. (1 nm) can be consistently made. The patent further discloses that millions of such features can be produced simultaneously on a wafer to simulate the effect of haze or microroughness on a polished wafer, and thereby be useful as a microroughness standard.
John C. Stover in an article entitled "Requirements and suggestions for industrial smooth surface microroughness standards" in SPIE vol. 2862 Proceedings, 8-9 August 1996, Denver, Colo., pp. 69-77, suggests a microroughness standard having a grating with a rectangular cross-section and a 50% duty cycle. With that standard the resulting power spectral density (PSD) is composed of an infinite series of harmonically related peaks (with even orders missing when the duty cycle is 50%) whose amplitudes relate to the height of the grating steps and which fall off as f.sup.-2, where f is the spatial frequency related to the pitch of the grating. Moreover, the RMS microroughness value is found by taking the square root of the sum of that series of peaks (out to about the 21st order).
In the case of step height standards, it has been found difficult to verify the step height in order to certify the standard when the step heights become less than about 50 .ANG.. For example, when atomic force microscopes are used to measure epitaxial silicon step height directly, the step height value obtained depends on the exact crystal orientation of the wafer and can easily be off by as much as 20%.
It is an object of the present invention to provide a method of certifying a step-height calibration standard with proved traceability and a method of calibrating step-height measuring instruments with the calibration standard certified by the first method.